1. Field of the Invention
The present invention relates in general to the field of electronics and lighting, and more specifically to a system and method to controlling and/or providing power to current regulated light sources, such as light emitting diode light sources.
2. Description of the Related Art
Commercially practical incandescent light bulbs have been available for over 100 years. However, other light sources show promise as commercially viable alternatives to the incandescent light bulb. LEDs are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output, long life, and environmental incentives such as the reduction of mercury.
LEDs are semiconductor devices and are driven by direct current. The brightness of the LED varies in direct proportion to the current flowing through the LED. Thus, increasing current supplied to an LED increases the brightness of the LED and decreasing current supplied to the LED dims the LED.
FIG. 1 depicts a switching light emitting diode (LED) driver system 100. The LED driver system 100 includes a continuous current mode, buck-based power converter 102 to provide a constant mains voltage Vmains to switching LED system 104. Voltage source 101 supplies an alternating current (AC) input mains voltage Vmains to a full, diode bridge rectifier 103. The voltage source 101 is, for example, a public utility, and the AC mains voltage Vmains is, for example, a 60 Hz/120 V mains voltage in the United States of America or a 50 Hz/230 V mains voltage in Europe. The rectifier 103 rectifies the input mains voltage Vmains. The hold-up capacitor C1 holds an approximately direct current (DC) supply voltage VC1 across capacitor C1 relative to a reference voltage VR. Supply voltage VC1 is also the output voltage of power converter 102 and the input voltage for controller 106. Input filter capacitor C2 provides a high pass filter for high frequency components of the output voltage of rectifier 103. A thermistor NTC 1 provides in-rush current protection for power converter 102.
The controller 106 is, for example, a Supertex HV9910B integrated circuit controller available from Supertex, Inc. of Sunnyvale, Calif. The supply voltage VC1 can vary from, for example, 8V to 450V. Controller 106 incorporates an internal voltage regulator to operate directly from the DC supply voltage VC. The controller 106 provides a gate drive signal from the GATE output node to the n-channel metal oxide semiconductor field effect transistor (MOSFET) Q1. Controller 106 modulates the gate drive signal and, thus, the conductivity of MOSFET Q1 to provide a constant current to switching LED system 104. Controller 106 modifies the average resistance of MOSFET Q1 by varying a duty cycle of a pulse width modulated gate drive signal VGATE. Resistor R1 and capacitor C3 provide external connections for controller 106 to the ground reference.
Controller 106 generates and uses feedback to maintain a constant current iLED. Controller 106 receives a current feedback signal Vfb representing a feedback voltage Vfb sensed across sense resistor R2. The feedback voltage Vfb is directly proportional to the LED current iLED in LEDs 108. If the feedback voltage Vfb exceeds a predetermined reference corresponding to a desired LED current, the controller 106 responds to the feedback voltage Vfb by decreasing the duty cycle of gate drive signal GATE to increase the average resistance of MOSFET Q1 over time. If the feedback voltage Vfb is less than a predetermined reference corresponding to the desired LED current, the controller 106 responds to the feedback voltage Vfb by increasing the duty cycle of gate drive signal VGATE to decrease the average resistance of MOSFET Q1 over time.
The switching LED system 104 includes a chain of one or more, serially connected LEDs 108. When the MOSFET Q1 is “on”, i.e. conductive, diode D1 is reversed bias and, current iLED flows through the LEDs and charges inductor L1. When the MOSFET Q1 is “off”, i.e. nonconductive, the voltage across inductor L1 changes polarity, and diode D1 creates a current path for the LED current iLED. The inductor L1 is chosen so as to store enough energy to maintain a constant current iLED when MOSFET Q1 is “off”.
FIG. 2 depicts a power control system 200, which includes a switching power converter 202. The rectifier 103 rectifies the input mains voltage Vmains and supplies a rectified, time-varying, primary supply voltage Vx to the switching power converter. The switching power converter 202 provides a power factor corrected, approximately constant voltage power to load 222.
PFC and output voltage controller 214 controls PFC switch 208 so as to provide power factor correction and regulate the output voltage Vc of switching power converter 202. The goal of power factor correction technology is to make the switching power converter 202 appear resistive to the voltage source 101. Thus, the PFC and output voltage controller 214 attempts to control the inductor current iL so that the average inductor current iL is linearly and directly related to the primary supply voltage Vx. The PFC and output voltage controller 214 supplies a pulse width modulated (PWM) control signal CS0 to control the conductivity of switch 208. In at least one embodiment, switch 208 is a field effect transistor (FET), and control signal CS0 is the gate voltage of switch 208. The values of the pulse width and duty cycle of control signal CSo depend on two feedback signals, namely, the primary supply voltage Vx and the capacitor voltage/output voltage Vc. Output voltage Vc is also commonly referred to as a “link voltage”.
To convert the input voltage Vx into a power factor corrected output voltage Vc, PFC and output voltage controller 214 modulates the conductivity of PFC switch 208. To regulate the amount of energy transferred and maintain a power factor close to one, PFC and output voltage controller 214 varies the period of control signal CS0 so that the input current iL tracks the changes in input voltage Vx and holds the output voltage VC constant. Thus, as the input voltage Vx increases, PFC and output voltage controller 214 increases the period TT of control signal CS0, and as the input voltage Vx decreases, PFC and output voltage controller 214 decreases the period of control signal CS0. At the same time, the pulse width (PW) of control signal CS0 is adjusted to maintain a constant duty cycle of control signal CS0, and, thus, hold the output voltage VC constant. The inductor current iL ramps ‘up’ when the switch 208 conducts, i.e. is “ON”. The inductor current iL ramps down when switch 208 is nonconductive, i.e. is “OFF”, and supplies inductor current iL to recharge capacitor 206. The time period during which inductor current iL ramps down is commonly referred to as the “inductor flyback time”. Diode 211 prevents reverse current flow into inductor 210. Inductor current iL is proportionate to the ‘on-time’ of switch 208. In at least one embodiment, the switching power converter 202 operates in discontinuous current mode, i.e. the inductor current iL ramp up time plus the inductor flyback time is less than the period of the control signal CS0, which controls the conductivity of switch 208. Prodić, Compensator Design and Stability Assessment for Fast Voltage Loops of Power Factor Correction Rectifiers, IEEE Transactions on Power Electronics, Vol. 22, No. 5, September 2007, pp. 1719-1729 (referred to herein as “Prodić”), describes an example of PFC and output voltage controller 214.
In at least one embodiment, the PFC and output voltage controller 214 updates the control signal CS0 at a frequency much greater than the frequency of input voltage Vx. The frequency of input voltage Vx is generally 50-60 Hz. The frequency 1/TT of control signal CS0 is, for example, between 20 kHz and 130 kHz. Frequencies at or above 20 kHz avoid audio frequencies and frequencies at or below 130 kHz avoids significant switching inefficiencies while still maintaining a good power factor of, for example between 0.9 and 1, and an approximately constant output voltage VC.
Capacitor 206 supplies stored energy to load 212 when diode 211 is reverse biased. The capacitor 206 is sufficiently large so as to maintain a substantially constant output voltage Vc, as established by a PFC and output voltage controller 214 (as discussed in more detail below). The output voltage Vc remains at a substantially constant target value during constant load conditions. However, as load conditions change, the output voltage Vc changes. The PFC and output voltage controller 214 responds to the changes in voltage Vc by adjusting the control signal CS0 to return the output voltage Vc to the target value. The PFC and output voltage controller 214 includes a small capacitor 215 to filter any high frequency signals from the primary supply voltage Vx.
PFC and output voltage controller 214 controls the process of switching power converter 202 so that a desired amount of energy is transferred to capacitor 206. The desired amount of energy depends upon the voltage and current requirements of load 212. To determine the amount of energy demand of load 212, the PFC and output voltage controller 214 includes a compensator 228. Compensator 228 determines a difference between a reference voltage VREF, which indicates a target voltage for output voltage Vc, and the actual output voltage Vc sensed from node 222 and received as feedback from voltage loop 218. The compensator 228 generally utilizes technology, such as proportional integral (PI) type control, to respond to differences in the output voltage Vc relative to the reference voltage VREF. The PI control processes the error so that the PFC and output voltage controller 214 smoothly adjusts the output voltage Vc to avoid causing rapid fluctuations in the output voltage Vc in response to small error signals. The compensator 228 provides an output signal to the pulse width modulator (PWM) 230 to cause the PWM 230 to generate a control signal CS0 that drives switch 208.
An LED lighting system controller, such as controller 106, using a supply voltage that can vary from, for example, 8V to 450V generally requires a more expensive integrated circuit relative to an integrated circuit designed to operate at a fraction of the maximum supply voltage. Using a conventional PFC controller with feedback control, when the power demand of a load quickly decreases, the output voltage VC will momentarily increase while the PFC controller responds to output voltage feedback by lowering the output voltage. Conventional switching power converters using compensators generally respond relatively slowly to large changes in load power demand. Additionally, conventional PFC controllers often include large and relatively expensive electrolytic capacitors to accommodate voltage spikes.